Simulating Electrochemical Impedance Spectroscopy of Graphite/LiNi0.8Co0.15Al0.05O2 Cathode Cells

Electrochemical systems have been studied extensively by electrochemical impedance spectroscopy (EIS).  Most often, EIS spectra are analyzed with equivalent electrical circuit models. These models combine a preexisting set of electrical components (resistors, capacitors, and constant phase elements) in series and parallel arrangements.  It is generally possible to get a good fit between the model and experimental data, but the correspondence between the physical processes in the cell and the circuit elements is sometimes unconvincing.

In this work, we apply a physics-based numerical approach that mimics the process used in some frequency response analyzers (FRA).  This FRA method is applied to the dual insertion cell model of Fuller et al.’s [1, 2] and incorporates expressions that describe the solid electrolyte interphase (SEI).  A schematic of the FRA implementation is shown in Figure 1.

The model can be used to simulate electrochemical impedance spectroscopy (EIS) by applying a sinusoidal current waveform of frequency, f, and integrating the potential response over an integer number of cycles, N. Impedance can be calculated from the attached equations [3].

EIS data were experimentally measured for LiNi_0.8Co_0.15Al_0.05O₂(NCA), half cells and full cells, at different states of charge (SOC).  A representative Nyquist plot is shown for an NCA half cell at 25 °C in Figure 2.

Simulation results are compared to experimentally obtained spectra and elucidate the physical processes that lead to distinct semicircles in the EIS spectra.  The model proves to be useful in gaining insight on battery life by relating capacity fade, cycling, and state of charge at open circuit voltage conditions.

REFERENCES

[1] Fuller TF, Doyle M, Newman J. Simulation and optimization of the dual lithium ion insertion cell. Journal of the Electrochemical Society. 1994;141:1-10.

[2] Spotnitz R. Simulation of capacity fade in lithium-ion batteries. Journal of Power Sources. 2003;113:72-80.

[3] Setzler BP, Fuller TF. Investigation of Transport Phenomena in PEMFC Through 1-D Macro-Homogeneous Model. ECS Transactions. 2013;58:95-107.